In the manufacture of semiconductor devices there are many process steps hich involve removal of silicon oxide. A number of known protocols for silicon oxide etch use a liquid or gas phase mixture of HF and a catalyst such as ROH where R is or alkyl (i.e. ROH=water or an alcohol). More recently gas phase systems using UV illuminated fluorine or fluorine interhalogen gas have also been described for this purpose.
It has long been known that gas phase HF/water mixtures can be used to etch various silicon oxide films. Early references include J. P. Holmes, et al, "A Vapor Etching Technique for the Photolithography of Silicon Dioxide," Microelectronics and Reliability, 5 pp 337-341 (1966); and K. Breyer, et al, "Etching of SiO.sub.2 in Gaseous HF/H.sub.2 O," IBM Technical Bulletin, 19(7) (12/1976), both of which used a HF/water azeotrope.
In U.S. Pat. No. 4,749,440 (Blackwood), a process for removing silicon oxide films from silicon wafers using anhydrous HF gas and water vapor carried in a nitrogen stream is disclosed. The gases are mixed just prior to entering a process chamber. The products are gaseous and are removed by the inert nitrogen carrier gas. This process has an advantage over previous liquid phase etching procedures in reduced heavy metals deposits, which are often introduced during rinse steps, and in reduced environmental problems. Additionally, the use of anhydrous HF provides improved process control compared to prior gas phase HF/water processes in which HF is supplied as an azeotrope with water. An ambient pressure apparatus for performing this process is currently commercially available from FSI International, Inc., under the trademark Excalibur.RTM..
Various publications describe HF/alcohol processes for etching silicon oxide.
Liquid phase reactions in which HF/alcohol mixtures are applied to a rapidly spinning silicon wafer under N.sub.2 in a glove box reaction chamber to remove silicon oxide are described in F. J. Grunthaner, et al, "Local Atomic and Electronic Structure of Oxide/GaAs and SiO.sub.2 /Si Interfaces Using High-Resolution XPS," J. Vac. Technol., 16 (5) 1443-1453 (1979); F. J. Grunthaner, et al, "Chemical and Electronic Structure of the SiO.sub.2 /Si Interface," Materials Science Reports, 69 (at 82-86 & 130-160) (1986); W. J. Kaiser, et al, "Scanning Tunneling Microscopy Characterization of the Geometric and Electronic Structure of Hydrogen-Terminated Silicon Substrates," J Vac. Soc. A, 6 (2), 519-523 (4/1988); and D. B. Fenner, et al, "Silicon Surface Passivation by Hydrogen Termination: A Comparative Study of Preparative Methods," J. Appl. Phys., 66(1) pp 419-424 (7/89). J. L. Prom, et al, "Influence of the Preoxidation Cleaning on the Electrical Properties of Thin SiO.sub.2 Layers," IEE Proceedings, Pt 1, 135, (1), 20-22 (2/88), reports use of a HF and ethanol (1:10) immersion as the last step in a preoxidation cleaning.
U.S. Pat. No. 5,022,961, (Izumi), describes a process for removing a film of a silicon oxide, from a silicon substrate. Two steps are identified:
(a) placing the substrate in a reaction chamber to be isolated and an air-tight manner from the outside air, and PA1 (b) feeding anhydrous hydrogen fluoride and alcohol simultaneously into the reaction chamber.
The reference indicates that the HF/alcohol feeds may be as liquid solutions or gas mixtures. A similar disclosure of an ambient pressure gas phase etch process is contained in A. Izumi, et al, "A New Cleaning Method by Using Anhydrous HF/CH.sub.3 OH Vapor System," J. Ruzyllo et al, ed., Symposium on Cleaning Technology in Semiconductor Device Manufacturing, ECS Proceedings, 92(12), pp 260-266 (1992).
U.S. Pat. No. 5,439,553 (Grant, et al), issued Aug. 8, 1995 from an application filed in the United States on Mar. 30, 1994, describes and claims a process for removing silicon oxide from a wafer substrate in which an HF/alcohol gas mixture is used at a low pressure to minimize condensation. The same process was earlier published in course materials distributed to attendees of a short course entitled "Semi-Conductor Wafer Cleaning Technology" which was held in Austin, Tex. on Feb. 23rd and 24th, 1993, by Werner Kern Associates, East Windsor, N.J. At that short course, one of the inventors of U.S. Pat. No. 5,439,553 also presented a lecture on dry cleaning processes which included a discussion of vapor phase etching of silicon oxide using a HF/methanol process under low pressure conditions where condensation does not occur.
J. Butterbaugh, et al, "Gas Phase Etching of Silicon Oxide with Anhydrous HF and Isopropanol," Proceedings of the Third International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, ECS Proceedings, 94(7) pp 374-383 (1994), describe a low pressure HF/isopropanol etch process for silicon oxide.
HF etching systems for silicon oxide removal which do not use added catalyst are reported in N Miki, et al, "Gas-Phase Selective Etching of Native Oxide," IEEE Transactions on Electron Devices, 37 pp 107-115 (1/90).
In EP 688045, a method for removing silicon oxides which is less selective for doped oxides than HF based processes is disclosed. The method employs fluorine or a gas photolyzable by UV irradiation to produce monoatomic fluorine species, together with UV irradiation of the substrate.
Several authors at Fujitsu Laboratories, Ltd., have produced publications describing a UV/Cl.sub.2 process for cleaning metal contamination from silicon wafer substrates and UV/F.sub.2 /Ar and UV/F.sub.2 /H.sub.2 processes for etching silicon oxide. These publications include T. Ito, "Wafer cleaning with photo-excited halogen radical," Proceedings--Institute of Environmental Sciences, 1991, pp 808-813; Aoyama et al, "Removing native oxide from Si(001) surfaces using photoexcited fluorine gas," Appl. Phys. Lett., 59, November 1991, pp 2576-2578; Aoyama et al, "Silicon Surface Cleaning Using Photoexcited Fluorine Gas Diluted with Hydrogen," J. Electrochem. Soc., 140, 1704-1708 (1993); Aoyama et al, "Surface Cleaning for Si Epitaxy Using Photoexcited Fluorine Gas," J. Electrochem. Soc., 140, 366-371 (1993); and U.S. Pat. No. 5,178,721.
As experience with these various gas-phase etch reactions has grown, it has become apparent that the results obtained can be highly variable depending on the handling history of the substrate, particularly with HF/catalyst etch systems. Etch depth and etch uniformity across the wafer may be quite reproducible if a wafer is quickly transferred from an oxide generating furnace to the etch reaction chamber but the reproducibility of the process can quickly deteriorate if the wafer is allowed to be exposed to a clean room environment, or to storage in a vacuum in standard plastic wafer cassettes, for as short as a few minutes. Careful control of the environmental exposure to uniform conditions can alleviate this problem to some extent, but sometimes the handling history is not known or is outside the normal controlled conditions established for a particular etch operation. For instance when a manufacturing sequence process is interrupted just prior to an etch operation due to unusual circumstances and cannot be resumed until sometime outside the time range allowed to initiate etch.
From the investigations leading to the invention it appears that a major source of etch non-uniformity may come from polymer cassettes used to store silicon wafers awaiting processing, particularly under vacuum storage conditions. Thus multiple wafers placed in a holding cassette at the same time after completion of an oxidation reaction and promptly transferred to a vacuum cluster tool apparatus for individual etch processing can display significantly different etch uniformity results depending on how long the wafer sits in the cassette awaiting its turn at the etch processing.
Because the handling history cannot always be controlled, or maintained identical from wafer to wafer, it would be desirable to be able to put a wafer into a condition prior to etching which allows the etch reaction to be reproducible without regard to the handling history. Further it would be desirable to be able to put a wafer into a passivated condition which will render the etch reaction less sensitive to handling history of the wafer between the time the wafer is passivated and the time it is etched. Still further it would be desirable to be able to accomplish these results on substrates of complex composition, including substrates which have areas of exposed silicon.